accounts.contacts: fix eval error

You would encounter an eval error when a module (such as pimsync) would
try to access an attribute of `accounts.contacts.<name>.local`, since it
would default to `null`. The same problem was encountered in the
`accounts.calendar` module, and fixed in
2c157e22dc which has the same solution.

Closes #8258

Reported-by: redbeardymcgee

modules/accounts/contacts.nix

@@ -102,8 +102,8 @@ let
         };
 
         local = mkOption {
-          type = types.nullOr (localModule name);
-          default = null;
+          type = localModule name;
+          default = { };
           description = ''
             Local configuration for the contacts.
           '';